appeared in the journal Nature Genetics._

Much of this discussion, Risch wrote in rebuttal of the two editorials, "does not derive from an objective scientific perspective." (In the determinedly dull parlance of the scientific literature, these are fighting words.) Numerous genetic studies of the human population have found that differences are greatest between continents. These studies, he said, "have recapitulated the classical definition of races based on continental ancestry." Updating those definitions, Risch and his colleagues suggested that racial groups should be defined on the basis of continent of origin, with ethnicity being used to describe smaller subdivisions within races.

The five continent-based races, in Risch's view, are as follows:^

Africans are those whose primary ancestry is in sub-Saharan Africa. This includes African Americans and Afro-Caribbeans.

Caucasians are people of western Eurasia—Europeans, Middle Easterners, North Africans and those of the Indian subcontinent (India and Pakistan).

Asians are people of eastern Eurasia (China, Japan, Indochina, the Philippines and Siberia).

Pacific Islanders are Australian aborigines and people of New Guinea, Melanesia and Micronesia.

Native Americans are the original inhabitants of North and South America.

Within each continental race there are gradations of skin color, from the light-skinned Khoisan speakers of southern Africa to the darker-skinned Bantu speakers of western and central Africa, from the lighter-skinned Scandinavians to the darker-skinned peoples of southern India. Skin color is therefore an ambiguous indicator of continental race. At the boundaries of these continental divisions are several groups formed by intermarriage between the two neighboring races, a condition for which geneticists use the term "admixture." Ethiopians and Somalis, for instance, are an admixture of Caucasians and Africans. "The existence of such intermediate groups should not, however, overshadow the fact that the greatest genetic structure that exists in the human population occurs at the racial level," Risch says. In the United States there are several populations formed by intermarriage between members of two racial groups. African Americans, largely as a result of slavery, have a share of Caucasian genes that ranges from 12% to 23% in various populations, with an average of about 17%. "Despite the admixture, African Americans remain a largely African group, reflecting primarily their African origins from a genetic perspective," Risch says.

Another group of admixed populations is counted by the U. S. Census Bureau as Hispanic although Hispanic is a linguistic, not a racial, category. Hispanics vary in their admixture in different parts of the country. In the southwestern United States, Hispanics are mostly Mexican Americans, whose ancestry is

39% Native American, 58% Caucasian, and 3% African, according to one recent estimate. East coast Hispanics come mostly from the Caribbean and have a larger proportion of African genes.

The United States is often referred to as a melting pot of races but the rate of mixing is slower than might be assumed. Figures from the 2000 U. S. census indicate that U. S. citizens do not marry each other at random. Racial endogamy (marrying within the racial group) is the rule: 97.6% of respondents reported themselves to be of one race; only 2.4% said they were of more than one race, presumably having parents of different races. Some 75% of Americans declared themselves to be white, that is, Caucasian; 12.3% said they were black or African American; 3.6% were Asian, 1% Native American, and 5.5% of other races.

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